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Re: xbd 32k transfer limit

On 5/3/2011, "Manuel Bouyer" <bouyer%antioche.eu.org@localhost> wrote:

>The real limit is inside the include/xen3-public/io/blkif.h file, which
>defines the interface between front and backend block devices.
>The size of a transfer is limited by BLKIF_MAX_SEGMENTS_PER_REQUEST, which
>is 11. With 4k pages, this makes a max transfer size of 44k. As we want
>a power of 2, we use 32k.
Well, no quick way around it, obviously. I bumped MAXPHYS to 64k and got
an assert in xbd_xenbus.c during bootup of the domu, so that's cool. I
checked xen 4.1 and it hasn't changed either since the reason for the
define is to keep the struct in a memory page boundary (4k). 

So, I guess the lesson is that for xen backing store on a 3 disk raid5
setup, stick to 32 sector stripe unit and 32k ffs block size. That is,
unless somebody actually implements scatter gather mechanism (wink,
wink, nudge, nudge).


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